The adsorptive properties of graphene oxide (GO) towards divalent metal ions (copper, zinc, cadmium and lead) were investigated. GO prepared through the oxidation of graphite using potassium dichromate was characterized by scanning electron microscopy (SEM), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy (FT-IR). The results of batch experiments and measurements by flame atomic absorption spectrometry (F-AAS) indicate that maximum adsorption can be achieved in broad pH ranges: 3–7 for Cu(II), 5–8 for Zn(II), 4–8 for Cd(II), 3–7 for Pb(II). The maximum adsorption capacities of Cu(II), Zn(II), Cd(II) and Pb(II) on GO at pH = 5 are 294, 345, 530, 1119 mg g⁻¹, respectively. The competitive adsorption experiments showed the affinity in the order of Pb(II) > Cu(II) ≫ Cd(II) > Zn(II). Adsorption isotherms and kinetic studies suggest that sorption of metal ions on GO nanosheets is monolayer coverage and adsorption is controlled by chemical adsorption involving the strong surface complexation of metal ions with the oxygen-containing groups on the surface of GO. Chemisorption was confirmed by XPS (binding energy and shape of O1s and C1s peaks) of GO with adsorbed metal ions. The adsorption experiments show that the dispersibility of GO in water changes remarkably after complexation of metal ions. After adsorption, the tendency to agglomerate and precipitate is observed. Excellent dispersibility of GO and strong tendency of GO–Me(II) to precipitate open the path to removal of heavy metals from water solution. Potential application of GO in analytical chemistry as a solid sorbent for preconcentration of trace elements and in heavy metal ion pollution cleanup results from its maximum adsorption capacities that are much higher than those of any of the currently reported sorbents.

We report the synthesis, crystal structure, and thermal, dielectric, phonon, and magnetic properties of [NH₂-CH⁺-NH₂][Mn(HCOO)₃] (FMDMn). The anionic framework of [(Mn(HCOO)₃^–] is counterbalanced by formamidinium (FMD⁺) cations located in the cavities of the framework. These cations form extensive N–H···O hydrogen bonding with the framework. The divalent manganese ions have octahedral geometry and are bridged by the formate in an anti–anti mode of coordination. We have found that FMDMn undergoes a structural phase transition around 335 K. According to the X-ray diffraction, the compound shows R3̅c symmetry at 355 K and C2/c symmetry at 295 and 110 K. The FMD⁺ cations are dynamically disordered in the high-temperature phase, and the disorder leads to very large bandwidths of Raman and IR bands corresponding to vibrations of the NH₂ groups. Temperature-dependent studies show that the phase transition in FMDMn is associated with ordering of the FMD⁺ cations. Detailed analysis shows, however, that these cations still exhibit some reorientational motions down to about 200 K. The ordering of the FMD⁺ cations is associated with significant distortion of the anionic framework. On the basis of the magnetic data, FMDMn is a weak ferromagnet with the critical temperature T_c = 8.0 K.

In this paper, graphene oxide/cellulose membranes were prepared in order to perform effective adsorption of heavy metal ions: cobalt, nickel, copper, zinc, cadmium and lead. Two types of membranes were fabricated, i.e. pressed and non-pressed membranes. The experiment showed that the pressed membranes are highly durable at different pH values, even in basic solutions, and they can be applied in separation/removal of heavy metal ions during vigorous shaking in the aqueous solution. The non-pressed membranes were proved to be less stable, however, they can be successfully applied in the filtration process at the high flow-rates. The results of the batch experiments and the measurements by the inductively coupled plasma atomic emission spectroscopy (ICP-OES) indicated that the maximum adsorption can be achieved at pH 4–8. Adsorption isotherms and kinetic studies indicated that the sorption of the metal ions on the membranes occurs in a monolayer coverage, hence it is controlled by the chemical adsorption involving the strong surface complexation of metal ions with the oxygen-containing groups on the surface of graphene oxide. The maximum adsorption capacity values of Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Pb(II) on the graphene oxide/cellulose membranes at the pH of 4.5 are 15.5, 14.3, 26.6, 16.7, 26.8, 107.9 mg g⁻¹, respectively. The competitive adsorption experiments showed the affinities of prepared membranes for the metal ions in the order of Pb > Cu > Cd > Zn ≥ Ni ≥ Co. The affinity order agrees with the first stability constant of the associated metal hydroxide and acetate. The adsorption properties of the graphene oxide/cellulose membranes, their reusability (more than 10 cycles) and durability in the aqueous solutions open the path to removal of heavy metals from water solution. The membranes can be also used in the field of analytical chemistry for the preconcentration and/or separation of trace and ultratrace metal ions.

We report the synthesis and studies of a novel heterometallic formate [C ₂ H ₅ NH ₃ ][Na _0.5 Fe _0.5 (HCOO) ₃ ].